Question

an object of height 1.2m is placed before a concave mirror of focal length 20cms so that a real image is formed at a distance of 60cm from it.find the position of an object .what will be the height of the image formed?

Answer 1

Formula:

(1/f)= (1/u)+(1/v)…………….(1)

Equation (1) is applicable to both concave mirror and convex mirror.

In our problem f and v are given and we are required to find u. From equation (1),

(1/u)=(1/f)-(1/v)=-(1/20)+(1/60)=-(1/30) OR

u=-30 cm.

For height of the image we use the following formula:

Magnification m= hi/ho=-v/u.( for concave mirror)

Here, ho=1.2m , v=-60cm and u=-30cm. Therefore ,

hi=ho(-60/30)=(1.2)(-2)=-2.4m. The negative sign indicates inverted image.

Answer 2

Answer:

u = 30 cm

$\display \text{h _i = 2.4 cm}$

Explanation:

We have given :

Focal length = 20 cm

Image distance =  60 cm

We have to find object distance .

We know :

1 / f = 1 / v + 1 / u

Since image formed is real :

f = - 20 cm and v = - 60 cm

1 / u = 1 - 3 / 60

u = - 30 cm

Hence object distance is 30 cm .

Also given object height = 1.2 cm

We know :

h_i / h_o = - v / u

h_i / 1.2 = - 60 / 20

h_i = - 2.4 cm

Hence image height is 2.4 cm .